Microwave applicator

ABSTRACT

The entrance and exit waveguide access tunnel means of a conveyorized microwave applicator are structured to accommodate the width of the conveyor belt and at the same time substantially prevent excess leakage of energy from the open ports. The cutoff frequency characteristics of the waveguide sections are controlled to reduce energy leakage and still permit the transverse of a wide belt. The reduction of energy leakage is further enhanced by the provision of energy absorbing media in the access means.

Edgar [111 3,749,874 1451 July 31,1973

[ MICROWAVE APPLlATOR Richard H. Edgar, Chelrnsford, Mass.

[75] Inventor:

[73] Assignee: Raytheon Company, Lexington, Ky. 22 Filed: June 2, 1972[21] Appl. No.: 259,333

5/1972 Schiffmann 219/1055 6/1969 Brosnatian et al. 333/95 X PrimaryExaminer-J. V. Truhe Assistant Examiner-Hugh D. Jaeger Attorney-HaroldA. Murphy et a1.

[57] ABSTRACT 521 vs. 01. 219110.55, 333/95 R wavegude means of aconveyorized microwave applicator are structured [51] Int. Cl. H051:9/06 to accommodate the width of the conveyor belt and at [58] Field ofSearch 219/1055; 333/95 R, h b n l k f 333/98 t 6 same time su stantia yprevent excess ea age 0 energy from the open ports. The cutoff frequencychar- [56] References cited acteristics of the waveguide sections arecontrolled to reduce energy leakage and still permit the transverse ofUNITED STATES PATENTS a wide belt. The reduction of energy leakage isfurther 3,551,199 12/1970 Forster 219/1055 enhanced by the provision ofenergy absorbing media Dench in the access means 3,365,562 1/1968Jeppson.... 219/10,.55 3,597,565 8/1971 1 Johnson 219/ 10.55 3 Claims, 5Drawing Figures MICROWAVE ENERGY 34 GENERATOR MICROWAVE APPLICATORIBACKGROUND OF THE INVENTION processing times, cleanliness, relativelylow cost and an absence of radiated heat during operation make microwaveheating more attractive in' home and industry. Most of the materialshave certain dielectric loss properties and essentially all of themicrowave-energy is absorbed by the product during the application ofenergy.

For the purposes of the present specification, the term microwave isconstrued to refer to electromagnetic wave energy having wavelengths inthe order of one meter to one millimeter and frequencies in excess of300 MHz. The generators most frequently employed in such applicators arethe magnetron, and high power vacuum tri'odes. The. assigned frequenciesof operation adjacent to the entrance and exit ports to absorb theescaping energy.

All the foregoing prior art teachings reflect the need in paststructures for widening the entrance and exit tunnel means toaccommodate the wider conveyor belts to prevent the material fromfalling off. This results in electrical propagating conditions differingfrom the deisred electrical characteristics of the rectangular waveguidemeans to attenuate any escaping energy.

SUMMARY OF THE PRESENT INVENTION The present invention provides forentrance and exit waveguide tunnel means abutting an oven enclosure inwhich a first portion dimension accommodates the width of thetransporting means. This width, however, is maintained over a heightdimension which is minimal to prevent any microwave energy leakage andstill allow for clearance of the conveyor belt. The second portionwaveguide means structure is now reduced in width to a dimension toprovide another energy propagating characteristic as near as possible tothe ideal cutoff frequency characteristic for preventing escape ofradiated energy in the oven enclosure. The access allocated by Federaland State regulatory bodies are I 915 and 2,450 MHz. The access meanscommunicating with the microwave oven enclosure handle TE modes ofpropagation where the cutoff frequency wavelength is equivalent to A, 2awhere 0 represents the width of a rectangular waveguide. V I

Conveyorized microwave applicator devices generally provide for conveyorbelts which must be wide enough to prevent the articles from falling offduring the traverse through the tunnel means and microwave heatingenclosure during operation. The width of the conveyor belt, therefore,imposes a mechanical limitation on the use of such equipment that theelectrical cutoff frequencies of the entrance and exit means cannot beas carefully controlled as possibleto prevent the escape of energythrough the open ports. Numerous solutions and structures are utilizedin the art to reduce the energy leakage. One example may be found inU.S. Pat. No. 2,868,939, issued Jan. 13, 1959, to R.V. Pound whichprovides for the placement of lossy energy absorbing panels inavestibule or tunnel adjacent to the main oven enclosure. Numerous lossymaterials are bonded to the panels to provide for energy absorption.

Other alternative prior art structures include the use of pluralquarter-wave blocking filters with adjusting members in each of theentrance and exit tunnel wave guide means as disclosed in U.S. Pat. No.3,048,686, issued Aug. 7, I962, to W. Schmidt. The use of slots in thewaveguide tunnel means have also been proposed,

with the slots having predeterminedwavelength orien-f tations to reflectany microwave energy attempting to escape through the tunnel means. InU.S. Pat. No. 3,624,335, issued Nov. 30, 1971, to E.C.'Dench andassigned to the assignee-of the present invention, slotted entrance andexit tunnel means are disclosd with lossy energy absorbing materialsmounted in such a manner as to cover all of the slot discontinuities.

A still further example of prior art'ingenuity may be found in U.S. Pat.No. 3,365,562, issued Jan; 23, 1968, to M.R. Jeppson, which provides forthe circulation of a dielectric medium within chambers or compartmentsmeans ports may also be providedwith high lossy microwave energyabsorbing material such as colloidal graphite suspension or othermaterials bonded to the walls or provided in separate panels. Asubstantial portion of the access means cross-sectional area is,therefore, structured not on the basis of the transporting means as istrue of all the prior art embodiments, but is ratherprincipally'dimensioned for the desired electrical parameters to preventthe escape of the microwave energy from the conductive heating-enclosureas well as to accommodate the transported material.

; BRIEF DESCRIPTION OF THE DRAWINGS trative embodiment taken along theline 2-2 in FIG.

FIG. 3 is a detailed cross-sectional view of a prior art waveguideaccess tunnel means;

FIG. 4 is a detailed cross-sectional view of an alterna tive embodimentof the invention; and

FIG. 5 is an isometric view of a portion of a micro-. wave applicatorhaving a E-plane band for the input of the microwave energy.

-DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2 ofthe drawings, a conveyorized microwave energy applicator 10*isillustrated. The oven enclosure 12 is formed by substantially'parallelconductive wall members. Access means withopen ports at the endscomprise entrance and exit waveguide tunnel means 14 and 16. Thematerials to be heated are transported through the oven enclosure bymeans of a conveyor belt 18 of a low dielectric loss material-carried bydrums 20 and'22. Generally the convenyor belt is formed of anonconductive material such as a plastic or leather composition and doesnot absorb microwave energy. Drum 22 is actuated by motor means 24 whichdrives a reducing gear mechanism 26 with the direction of travel of thebelt indicated by the arrows 28.

Electromagnetic energy at the assigned frequencies of either 9 l or 2450MHz is illustrated as radiating the enclosure 12 through a suitableaperture in top wall 30 by means of a rectangular waveguide section 32.The microwave energy source includes a generator, such as a magnetron,together with all the electrical and safety controls indicated generallyby box 34. No specific details of the energy source has been referred inview of 5 the belief that such means are now considered to be well knownin the art.

The entrance and exit waveguide tunnel means 14 and 16 will now bedescribed with attention being directed to FIG. 2. Conveyor belt 18 hasa predetermined width to accommodate the articles to be transportedthrough enclosure 12. The first portion of the rectangular waveguidetunnel 14 is, therefore, dimensioned to accommodate the width of thebelt 18. As a result, this region of the access means has a first energypropagating characteristic. The height of this portion designated by theletter H is maintained as small as possible in order to prevent theescape of microwave energy as well as to accommodate the transportingmeans. The remainder of the waveguide means 14 has a reduced widthdesignated by the letter W and a height H to result in a secondelectrical propagation characteristic for any escaping energy and toaccommodate the transported articles. Desirably the dimensions of thesecond or upper waveguide portion provided by walls 36 and 38 has acutoff frequency wavelength as near to escaping radiated energy or )t/2. Exit waveguide tunnel means 16 is similarly dimensioned.

Referring now to FIG. 3 the advantages of-thev present invention overprior art embodiments will be evident. Rectangular waveguide section 40comprises broad upper and lower waveguide walls 42 and 44 and narrowsidewalls 46 and 48. The dimensions of the rectangular waveguide areuniformthroughout and are determined by the width of the conveyorbelt'50. The invention, therefore, provides for plural cross-sectionalareas of the waveguide access means of varying dimensions with theprevention of the escape of radiated energy as the primary considerationrather than the transporting means. The articles to be heated in themicrowave applicator of the invention are still prevented from fallingoff the conveyor belt during traversal and the escape of radiated energyis substantially reduced.

Referring to FIG. 4 a means for the further attenuation of any escapingenergy will be described. Waveguide section 52 constructed in accordancewith the teaching of the invention is provided on all of the insidewalls with an energy'absorbing medium such as any of the carbonaceousproducts coated on such walls or impregnated in panels of insulatingmaterials to form a layer 54. The absorbing medium may be cemented tothe metallic walls of the waveguide 52 by such means as a thermallyconductive cement to insure the conduction of themial energy resultingfrom the absorption of the microwave energy.

In FIG. 5 the application of the invention to other waveguide microwaveenergy applicator structuresis shown. An input section 56 of rectangularwaveguide having a substantially E-plane band is adapted in accordancewith the invention with waveguide access tunnelumeans 58 introducedthrough an opening in bend portion 60 of the waveguide. Theconveyor-beltaccommodating portion is provided by the lower portion 62and defined by walls 64 and 66. The upper portion of the waveguide hasthe narrower upper wall'68 and sidewalls 70.

The access tunnel waveguide means disclosed herein will be equallyapplicable to any microwave energy applicators having open ports for theintroduction of the material to be heated, particularly those of theconveyorized type; Such other variations; modifications and alterationswill be evident to those skilled in the art. It is intended, therefore,that the foregoing detailed description of the invention and preferredembodiments be considered in the broadest aspects and not in a limitingsense.

I claim:

' l. Conveyorized prising:

a source of electromagnetic energy microwave heating apparatus comaconductive enclosure with energy coupling means;

access tunnel means coupled to opposing ends of said enclosure;

means for transporting articles through said enclosure and access meansenclosure;

said access means having superimposed portions of differentcross-sectional dimensions throughout their length;

a first of said portions being dimensioned to accom-' modate saidtransporting means; and

the second portion is dimensioned to provide a cutoff frequencywavelength characteristic to substantially prevent escape of energyand'accommodate said transported articles.

2. The applicator according to claim 1 wherein the inner walls of saidaccess means are lined with an energy absorbing material.

3. The apparatus according to claim 1 wherein said energy coupling meanscomprise a rectangular waveguide having a substantially right angularbend and said access means are disposed at said bend.

1. Conveyorized microwave heating apparatus comprising: a source of electromagnetic energy; a conductive enclosure with energy coupling means; access tunnel means coupled to opposing ends of said enclosure; means for transporting articles through said enclosure and access means enclosure; said access means having superimposed portions of different cross-sectional dimensions throughout their length; a first of said portions being dimensioned to accommodate said transporting means; and the second portion is dimensioned to provide a cutoff frequency wavelength characteristic to substantially prevent escape of energy and accommodate said transported articles.
 2. The applicator according to claim 1 wherein the inner walls of said access means are lined with an energy absorbing material.
 3. The apparatus according to claim 1 wherein said energy coupling means comprise a rectangular waveguide having a substantially right angular bend and said access means are disposed at said bend. 